Recently, telecommunications such as mobile communication and satellite communication have focused a growing interest in dielectric ceramic devices for microwave application. In particular, mobile communication arrangements which include automobile telephones, cellular phones, pagers and GPS(Global Positioning Systems) employ microwave dielectric materials which are required to possess various electrical and physical properties such as, for example, high permittivity(.di-elect cons.), high quality factor(Q), small temperature coefficient of resonance frequency(T.sub.f), and good sintering characteristics.
Studies pertaining to dielectric compositions for microwave application have been directed to dielectric compositions of the TiO.sub.2 type. As the result, it has been determined that TiO.sub.2 based dielectric compositions including Ba.sub.2 Ti.sub.9 O.sub.20, (Zr,Sn)TiO.sub.4, BaO--Re.sub.2 O.sub.3 --TiO.sub.2 (Re:Rare earth) and BaO--Nd.sub.2 O.sub.3 --TiO.sub.2 (BNTtype) aong with dielectrics having a complex perovskite structure such as Ba (Mg.sub.1/3 Ta.sub.2/3) O.sub.3, Ba (Zn.sub.1/3, Ta.sub.2/3) O.sub.3 and Ba (Mg.sub.1/3 Nb.sub.2/3) O.sub.3 are suitable for the above type of application. Further, efforts are being actively directed to the development of new dielectric materials which are formed using solid solutions of two or more ceramic compositions having perovskite structures.
The dielectrics of the BNT ,type, however, present problems in that they have Q factor smaller than other dielectrics with respect to high frequency and exhibit a limited resonance frequency below 1GHz. Further, Nd.sub.2 O.sub.3 is a rare-earth metal which is a costly compared with other elements.
With regard to dielectrics of (Zr,Sn)TiO.sub.4 type, which are widely used due to their high Q factor and stable temperature characteristics, permittivity is in the range of 30 to 40, Q factor is about 8000 at 4 GHz and temperature coefficient of resonance frequency is in the range of -30 to +30ppm/.degree. C. This composition which is manufactured by a general solid phase reaction, is, however, sintered at a sintering temperature above 1600.degree. C., and is difficult to sintered at low temperatures without the addition of sintering agent such as CuO, Co.sub.2 O.sub.3, ZnO, and the like. However, the addition of the sintering agent deteriorates the physical properties of the ceramic composition.
Although various liquid phase methods are used for powder syntheses (e.g. Sol-Gel, alkoxide and coprecipitation methods), these methods are too complex to carry out economically and result in the rise of production costs.
Dielectrics of complex perovskite type ceramic compositions, for example, Ba(Zn.sub.1/3 TA.sub.2/3)O.sub.3, are also difficult to sinter because of a sintering temperature above 1550.degree. C. Furthermore, it is difficult to control the numerous process factors in case that agents such as BaZrO.sub.3 and Mn are added to decrease the sintering temperature.
As the size of electronic equipment such as dielectric filters becomes smaller, multilayer devices have been considered in connection with the necessary miniaturization. However, the production of such arrangements requires that the dielectric material and the electrodes be co-fired. In order to use low-priced Ag or Cu electrodes, the dielectric material must exhibit a low sintering temperature. For this reason, it still remained necessary to develop new dielectric ceramic compositions which had good sintering characteristics and simple compositions as well as at least still having the characteristics of conventional dielectric ceramic compositions necessary for microwave applications.
In compliance with the necessity, several dielectric ceramic compositions have been disclosed so far. For example, Glass is added to the dieclectric ceramic compositions of BaO--PbO--Nd.sub.2 O.sub.3 --TiO.sub.2 type (sintering temperature of 1300.degree. C.)it results in the drop of the sintering temperature to 900.degree. C. These compositions possess permittivity of 67, high quality Q factor of 570 at 5.1 GHz, and temperature coefficient of resonance frequency of 20 ppm/.degree. C. Besides, glass is added to dielectric compositions of CaZrO.sub.3 type(sintering temperature of 1350.degree. C) to lower the sintering temperature to 980.degree. C., and the dielectric compositions possess permittivity to 25, Q factor of 700 at 5.1 GHz, and temperature coefficient of resonance frequency of 10 ppm/.degree. C.
In addition, U.S. Pat. No. 5,756,412 invented by the present inventors discloses dielectric ceramic compositions of ZnNb.sub.2 O.sub.6 to which a sintering agent such as CuO, V.sub.2 O.sub.5, Bi.sub.2 O.sub.3, Sb.sub.2 O.sub.3, and the like, is added and thus sintering temperature is lowered to below 900.degree. C.
Even though the ceramic compositions of ZnNb.sub.2 O.sub.6 type have good dielectric characteristics, the temperature coefficient of resonance frequency is relatively large (negative value), so there is a limit to actual applications for dielectric materials.
The present invention is an improvement over U.S. Pat. No. 5,756,412, and since TiO.sub.2 possess good dielectric characteristics such as high permittivity and high Q factor and significantly large positive temperature coefficient of resonance frequency, ZnNb.sub.2 O.sub.6 is mixed with TiO.sub.2, in proper mole fraction so that the inventive dielectric compositions exhibit temperature coefficient of resonance frequency that may be controlled in a proper range, and good sintering characteristics is still maintained in desirable levels.